This invention relates to the use of certain small particle catalysts in a slurry hydrotreating process for the removal of sulfur and nitrogen compounds and the hydrogenation of aromatic molecules present in light fossil fuels such as petroleum mid-distillates.
A well known application for a hydrotreating process in a refinery is the treatment of the light catalytic cracker cycle oil (LCCO) product from a catalytic cracker. The term LCCO may refer to furnace oil, diesel oil, or mixtures thereof, as distinguished from the other main product streams of the catalytic cracker, typically the gasoline and gas product stream and the heavy fuel oils product stream.
The LCCO product is relatively high in aromatic content and increasingly so as a result of the catalytic cracker being operated at a higher temperature in order to produce more gasoline. In other words, a higher gasoline conversion in the catalytic cracker is being obtained at the expense of a more aromatic LCCO product than in the past. However, the LCCO product is generally of less demand and consequently of less value than the gasoline product, and the problem of disposing of the LCCO product has arisen. One option is to hydrogenate the aromatics in the LCCO product and sell it as heating oil. However, this option may not be viable when the market for heating oil is insufficient. A second option is to make the LCCO product suitable for diesel oil stock. However, there already exists a stringent sulfur limit for diesel fuel and there is likely to be a stringent aromatics limit because of the effect of aromatics on soot formation. A third option for the LCCO product is to recycle it back to the catalytic cracker for further conversion, but since coke making is to be avoided, it is necessary to hydrogenate the LCCO before recycling.
The petroleum industry therefore hydrotreats LCCO's such as furnace oil or diesel oil, whether to upgrade the same for a final product or to upgrade them for recycle to the catalytic cracker.
Hydrotreating is a process wherein the quality of a petroleum feedstock is improved by treating the same with hydrogen in the presence of a hydrotreating catalyst. Various types of reactions may occur during hydrotreating. In one type of reaction, the mercaptans, disulfides, thiophenes, benzothiophenes and dibenzothiophenes are desulfurized. The thiophenes, mercaptans and disulfides are representative of a high percentage of the total sulfur in lighter naphthas. Benzothiophenes and dibenzothiophenes appear as the predominant sulfur forms in heavier feeds such as LCCO and VGO. Hydrotreating also removes nitrogen from various nitrogen compounds such as carbazoles, pyridines, and acridines. Hydrotreating can also hydrogenate aromatic compounds, existing as condensed aromatic ring structures with 1 to 3 or more aromatic rings such as benzene, alkyl substituted benzene, naphthalene, and phenanthrene.
The most common hydrotreating process utilizes a fixed bed hydrotreater. A fixed bed system, however, has several disadvantages or inherent limitations. At relatively low temperatures and employing a conventional catalyst, a fixed bed system is characterized by relatively low reaction rates for the hydrogenation of multi-ring aromatics and the removal of nitrogen in the material being treated. On the other hand, at relatively higher temperatures, a fixed bed system suffers from equilibrium limits with respect to the degree of aromatics hydrogenation.
Another limitation of a fixed bed system is the difficulty in controlling the temperature profile in the catalyst bed. As a result, exothermic reactions may lead to undesirably higher temperatures in downstream beds and consequently an unfavorable equilibrium. Still a further limitation of a fixed bed system is that a high pressure drop may be encountered, when employing small particle catalysts to reduce diffusion limits. Finally, a fixed bed system suffers from catalyst deactivation, which requires period shutdown of the reactor.
Hydrotreating processes utilizing a slurry of dispersed catalysts in admixture with a hydrocarbon oil are generally known. For example, U.S. Pat. No. 4,557,821 to Lopez et al discloses hydrotreating a heavy oil employing a circulating slurry catalyst. Other patents disclosing slurry hydrotreating include U.S. Pat. Nos. 3,297,563; 2,912,375; and 2,700,015.
Conventional hydrotreating processes utilizing a slurry system avoid some of the limits of a fixed bed system. In a slurry system, it is possible to use small particle catalysts without a high pressure drop. Further, it is possible to replace deactivated catalyst "on-stream" with fresh reactivated catalyst. However, the conventional slurry hydrotreating process at high reactor temperatures still is limited with respect to the overall degree of aromatics hydrogenation. At low temperatures, it is possible to obtain better heat transfer and mixing and to control any temperature rise so as to maintain a favorable equilibrium level. However, the overall reaction rates in the conventional slurry process at low temperatures are relatively poor. Poor reaction rates are believed to result from poisoning of the catalyst by organic nitrogen molecules in the feed being treated. Such compounds adsorb on the catalyst and tie up the sites needed for hydrotreating reactions.
The present process overcomes the limits and disadvantages of conventional hydrotreating by employing certain finely divided hydrotreating catalysts in slurry form to contact the feed. According to the present invention, sufficient catalyst sites are packed into the slurry such that most of the nitrogen molecules can be titrated, that is absorbed, on the slurry catalyst without adversely affecting the hydrotreating process. Excess catalyst sites are present such that sites free of nitrogen are capable of hydrogenating the aromatics in a low or essentially nitrogen free feed.
The hydrotreating process of the present invention has the advantage that it can occur even at low temperatures, for example 650.degree. F to 700.degree. F, where equilibrium is favorable. In a further aspect of the present invention, any nitrogen is subsequently removed from the catalyst in a high temperature reactivation step before the catalyst recontacts fresh feed.